1. Field of the Invention
The present invention relates to a surface active agent and coating compositions containing the same to be used in the field of coating paints or laminates with high uniformity and high surface smoothness by a spray coating, and in the field of coating a thin coat of a photo-resist containing a photo-sensitizer by a spin-coating for the production of semiconductor devices, such as ICs, LSIs, liquid crystal display devices, and thermal heads.
2. Background Art
Conventionally, various surface active agents, called leveling agents, in chemical compositions of hydrocarbon, silicone, and fluorine are used in the field of coating in order to improve the uniformity and smoothness of the coated layers. Among various surface active agents, fluorine surface active agents are widely used due to their high capability in reducing the surface tension of the coating composition and in coating without leaving contamination after coating.
However, in a high speed coating or in a coating operation under high shearing force such as a spin coating or a spray coating, the coating composition containing a surface active agent which is capable to reduce only the static surface energy (the definition will be given hereinafter) does not necessarily produce sufficient leveling of the coated layer, causing pin-holes, craters, and fish-eyes on the coat surface, which result in deteriorating the product values. Typical examples of such high load coatings are the spray coating of a paint or the spin coating of a photo-resist on a silicon substrate in the manufacturing process of semiconductor devices. In the spray coating, the liquid paint is converted into droplets at the moment when the paint leaves the spray nozzle, which results in a drastic increase of the surface area. If these droplets of paint do not contain sufficient surface active agent at the time when droplets are formed, the resin particles which are the main component of the paint are exposed on the droplet surface during coating, which results in poor leveling of the coated film by causing craters, pin-holes, and fish-eyes at the film surface. The conventional leveling agent used in the spin-coating tends to cause an uneven surface finish called "striation" which is a fatal defect in the field of micro-machining.
A few surface active agents and the coating compositions containing these surface active agents are disclosed in Japanese Patent Application, First Publication, No. Hei-3-30825, and Japanese Patent Application, First Publication, No. Hei-8-62834. These documents disclose that these surface active agents are effective to some extent, but it is not clear how these agents are used in practical coating operations.
Conventionally, it is known that there is a relationship between the leveling of a coated layer and the surface tension of the coating composition. However, it is observed that a dynamic surface tension of a coating composition is an important factor to determine the leveling of a coated layer when the coating is carried out at high speed and under high shearing force.
The surface tension of a pure liquid reaches an equilibrium value within a period of less than 0.05 sec or presumably within 10.sup.-9 seconds, after a new surface is formed. However, the surface tension of a water solution or a solution containing a surface active agent decreases gradually after a new surface is formed and reaches a constant value after several hours or even after even a few days. This type of changing surface tension is defined as the "dynamic surface tension", while constant surface tension is defined as the "static surface tension". The surface tension is an important factor in operations of emulsification, dispersion, foaming, wetting, and rinsing, and, conventionally, merely the static surface tension of the solution was measured to prepare for these operations. However, it is clear that the above surface phenomena are related to dynamic surface tension as reported by Sasaki in"Surface", vol. 17, No.2, p.138 (1979). Further, it was shown that the effect of the dynamic surface tension should be negated in order to eliminate surface defects of a coating layer, as disclosed by G. P. Bierwagen in Prog. Org. Coatings, No. 3, p. 101 (1975).
Therefore, the object of the present invention is to provide a surface active agent containing fluorine and coating compositions using the same, which offer excellent leveling of a coated layer and the coated film by drastically reducing dynamic surface tension, even when the coating is performed under high load coating operations at a high speed and under a high shearing force.